In 1862, in the middle of the Civil War, the House Judiciary Committee looked into how a message from President Abraham Lincoln had leaked to the press. Rumor had it that first lady Mary Lincoln had passed it to a middleman, but the middleman refused to say. Then a surprise witness showed up to answer the committee’s questions: Lincoln himself. The leaker was soon identified as a White House gardener.

Lincoln apparently testified voluntarily, but other presidents and ex-presidents have received subpoenas compelling them to testify before Congress, in civil and criminal proceedings, and even before a military tribunal. Former president Donald Trump joined this group Thursday when the House Jan. 6 committee voted unanimously to subpoena him.

Law professor Ronald D. Rotunda, who died in 2018, served on the Senate committee investigating President Richard M. Nixon in the 1970s and dug deep into the history of presidents and subpoenas. Here’s what he found.

In 1846, the House began investigating a former secretary of state, Daniel Webster, who had served under presidents John Tyler and John Quincy Adams, for misuse of funds. The sitting president, James K. Polk, gave a limited amount of information about the funds to the House but didn’t think it would be proper to “publicly reveal confidences of his predecessors,” according to Rotunda.

Congress convened two select committees and subpoenaed both former presidents. Tyler complied, testifying before both committees, and Adams filed a deposition. James Buchanan, a future president who was secretary of state at the time, was also subpoenaed and testified. Webster was cleared of wrongdoing.

In 1953, former president Harry S. Truman refused to comply with a House subpoena after he was accused of knowingly appointing a Russian spy to a position while in office. The House Un-American Activities Committee, vociferously engaged in interrogating anyone and everyone for alleged communism, subpoenaed Truman. He refused to comply, instead giving a national broadcast denying the accusation. The House didn’t push the matter.